net: phy: meson-gxl: add read and write helpers for banked registers

Add read and write helpers to manipulate banked registers on this PHY
This helps clarify the settings applied to these registers and what the
driver actually does

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>